The present invention generally relates to fabrication of semiconductor devices and more particularly to a method for growing a dielectric film by means of a vapor phase process and a fabrication process of a semiconductor device having a dielectric film thus formed.
With development of fine lithographic patterning processes, large capacity semiconductor memory devices having a storage capacity exceeding 256 Mbits or 1 Gbits are studied intensively, particularly in relation to DRAMs. In such large capacity DRAMs, on the other hand, there emerges a problem in that memory capacitors, which are used in DRAMs for storing information in the form of electric charges, cannot provide sufficient electric capacitance due to excessive miniaturization of the device and hence the capacitor area. Thus, it is necessary in such a large capacity semiconductor memory devices to increase the height of the miniaturized memory cell capacitors in order to secure sufficient capacitance, while use of such a tall capacitor inevitably invites a difficulty in conducting exposure by means of high resolution optical system that has a characteristically limited focal depth. It should be noted that there appears a step on the substrate of such a semiconductor device between the memory cell region in which the tall memory cell capacitors are formed and the peripheral circuit region in which no tall capacitors are formed.
In order to circumvent such a problem, there is a proposal to form the memory cell capacitor in the form of stacked fins such that the surface area of the memory cell capacitor is increased without increasing the height of the capacitor. However, such an approach is successful only for those memory devices having a storage capacity of less than 256 Mbits. When the storage capacity exceeds 256 Mbits, it is difficult to suppress the increase in the height of the memory cell capacitor.
In view of such a restriction in the structure, current DRAMs are generally constructed to have extremely reduced thickness for the dielectric film formed of SiO.sub.2 or SiN, in the order of several nanometers or less, such that the capacitance of the memory cell capacitor is maximized. On the other hand, further reduction in the thickness of the dielectric film is difficult in view of possible formation of pinholes or other defects in such thin dielectric films. Further, there is a possibility that tunneling current may flow through such thin dielectric films to cause dissipation of the accumulated electric charges and hence information.
Thus, it has been difficult in the conventional large capacity DRAMs of 256 Mbits or 1 Gbits to have a memory cell capacitor of which capacitance exceeds several femto Farads (fF).
Under such a situation, there is a demand for a technology that enables deposition of dielectric films having a dielectric constant substantially larger than the dielectric constant of the dielectric films currently used in the fabrication of semiconductor memory devices. Particularly, use of strontium titanate (SrTiO.sub.3), barium titanate (BaTiO.sub.3), or their solid solution ((Sr, Ba)TiO.sub.3) is thought promising in view of very large dielectric constants of these materials. Further, the technology for forming a film of SrTiO.sub.3 or other dielectric films of Sr is not only useful in the fabrication of semiconductor memory devices but also in the fabrication of novel transistors such as dielectric base transistor described in the U.S. Pat. No. 5,291,274 or high temperature superconductors such as Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.X.
In the fabrication semiconductor devices, it is particularly preferable and advantageous to deposit the dielectric film of Sr by a vapor phase process such as CVD process. In the conventional CVD process of dielectric films that contain Sr, Werner complexes such as Sr(thd).sub.2 (2,2,6,6-tetramethyl-3,3-heptanodionato strontium) known also as Sr(DPM).sub.2 (bis(dipivaloylmethanato)strontium) or Sr(HFA).sub.2 (hexafluoloacetylacetonato strontium) have been used as a source material. As these substances are solid at the room temperature, these materials are heated when conducting a CVD process to procure a gaseous source material. The gaseous source material thus obtained is then supplied to a reaction chamber for deposition on a substrate.
In such a heating process, it is necessary to heat the source material to a temperature of 200.degree. C. or more in correspondence to the high sublimation temperature of these substances. It should be noted that Sr(thd).sub.2 has a sublimation temperature in the range between 200-250.degree. C. This means that the gaseous source material thus produced has to be held at 200.degree. C. or higher until it reaches the reaction chamber and that a specially constructed piping system or line be used for supplying such a high temperature source gas.
In the conventional CVD process of these dielectric films, the solid source material is once dissolved into an organic solvent, wherein the desired gaseous source material of Sr is obtained from the liquid source material thus formed. For example, the formation of a SrTiO.sub.3 film from Sr(thd).sub.2 may be started with a process of dissolving Sr(thd).sub.2 into a solution of THF (tetrahydrofuran) with a concentration of 0.01-0.1 mol/l, followed by formation of gaseous source materials from the liquid source material thus formed. Thereby, it should be noted that one cannot obtain necessary vapor pressure for deposition of the dielectric film as long as the vaporization of the gaseous source material is conducted at the room temperature. Thus, the vaporization of the gaseous source material has to be conducted at a temperature of 200.degree. C. or more. Conventionally, the liquid source material is therefore supplied to a preheat furnace held at 200.degree. C. by means of a pump while controlling the flowrate of the same, for causing a vaporization of the gaseous source material. The gaseous source material thus vaporized in the preheat furnace is then admixed with tetraisopropoxy titanium (Ti(o-i-C.sub.3 H.sub.7).sub.4), a source of Ti, as well as with oxygen in a mixing chamber and is further forwarded to a reaction chamber, wherein a film of SrTiO.sub.3 is deposited on a substrate held at a temperature of 600.degree. C.
In such a conventional process that includes the step of heating the liquid source material of Sr(thd).sub.2 in a preheat furnace at 200.degree. C. prior to the deposition, it should be noted that the temperature of the gaseous source material thus formed has to be maintained within the range of 200-205.degree. C. in order to avoid deposition of material in the line that extends from the preheat furnace to the reaction chamber as well as to achieve the desired vapor pressure. When the temperature decreases below 200.degree. C., the gaseous source material of Sr(thd).sub.2 may cause a condensation. On the other hand, such a requirement indicates that one has to use a complex temperature control system in addition to the use of refractory material and construction for the line.
It should be noted that heating of Sr(thd).sub.2, either in the form of solid or in the form of liquid dissolved in an organic solvent, may cause a deterioration of in Sr(thd).sub.2 as a result of reaction with excessive thds contained in the source material or other impurities such as O.sub.2, CO.sub.2, CO, H.sub.2 O, and the like, without forming Sr(thd).sub.2. When such a deterioration occurs in the Sr(thd).sub.2 source material, the source material may not vaporize completely, even when heated to a predetermined temperature. Further, use of such deteriorated source material may result in a change in the vapor pressure of the gaseous source material produced as a result of vaporization.
Because of this, conventional Sr(thd).sub.2 source materials cannot be used more than several times and frequent replacement of the source material has been needed. However, the source material of Sr(thd).sub.2 that is available at present generally has a purity that changes in a wide range, and it has been needed to optimize the deposition condition each time a new source material is loaded. Because of this, it has been difficult to fabricate the semiconductor devices that use a dielectric film of Sr in a production line.